Recently, as a memory device that replaces a HDD (Hard Disk Drive), a so-called SSD (Solid State Drive) that uses a flash memory for a storage medium receives attention (see, for example, Japanese Laid-open Patent Publications No. H5-150913, No. H6-4399, and No. 2007-41813).
Since the SSD does not have a magnetic disk like the HDD, a time for seek operation to move a head to a desired track and a time until when a desired data position in the track to reach the head are unnecessary. Therefore, with the SSD, it is possible to perform storage and reproduction of data faster than the HDD, and thus the SSD receives attention. Further, the SSD does not need to rotate a magnetic disk by using a motor or move the head as opposed to the HDD, resulting in low power consumption, which is another reason of receiving attention.
Here, most of the SSD is provided with a DRAM (Dynamic Random Access Memory) for buffering when storing data in a flash memory. Recently, the SSD is demanded of further speedup and as a method, a capacity increase of the DRAM for buffering is considered.
On the other hand, the SSD is demanded of further power-saving. However, the DRAM has large power consumption in many cases, and the power consumption becomes larger as a capacity of the DRAM increases. Because of this, currently, achieving further speedup while saving electric power of the SSD is difficult.
Incidentally, up to here, a problem that achieving high-speed and power saving at the same time is difficult is explained by taking the SSD as ways of example. However, this problem may generally occur in a memory system that uses a storage medium having large power consumption like the DRAM as a buffer to store data.